This invention relates to optical fiber transmission systems, and more particularly to optical fibers that are constructed to prevent theft of optical signals.
Unauthorized and undetected tapping of optical signals from optical fiber transmission lines is a concern of a variety of users. Transmission cables, even those carrying highly sensitive information, often extend across substantial lengths of insecure territory. If an optical fiber is actually cut, and spliced into an eavesdropping device, the potential presence of an unwanted listener is easily detected due to the interruption in the signal at the receiving station. However, more sophisticated techniques are available that allow tapping of a portion of the signal without interruption with such a small loss of power that the tapping goes undetected. One such technique involves forming a refractive index grating in the core of the fiber. Typically, the optical fiber in which the grating is written has a dopant that is sensitive to photoinduced refractive index change. Once written, the grating diffracts a portion of the signal wavelength through the side of the fiber, where it is then easily xe2x80x9creadxe2x80x9d by a photodetector located next to the fiber. The grating is produced by exposing a short length of the fiber core to suitable laser radiation to create bands of refractive index perturbations. This can be done without enough compromise of the bulk transmission characteristics of optical fiber to be detected.
Another technique, simpler in concept, is to form a bend in the fiber. This causes xe2x80x9cleakingxe2x80x9d of the signal into the optical fiber cladding where it can be intercepted without detection from the source or receiving stations.
The first case of intrusion is dealt with according to the invention by constructing the optical fiber with a highly absorbing UV coating. This prevents access of the core of the fiber to the xe2x80x9cwritingxe2x80x9d radiation necessary to form the grating. In one variation of this embodiment, one or more additional optical paths are provided in the optical fiber for monitoring signals. The added optical paths allow monitoring signals to be transmitted in the optical fiber, separate from the information signal, to signal any attempt to breach the cladding of the optical fiber.
The second case of intrusion is addressed by increasing the sensitivity of the optical fiber to microbending loss to the extent that bends in the fiber cause such high attenuation of the signal that the bends do not go undetected at the receiving station.